This invention relates to a system for controlling a safety device for a vehicle, such as an air bag.
As discussed in Japanese Unexamined Patent Publication No. 79,450/91, a typical conventional system for controlling an air bag comprises a first and a second transistors (switching means) connected in series between a power source and the ground, and a squib (actuating element) connected in series between the first and second transistors. This control system further comprises an acceleration sensor and a microcomputer. In accordance with an acceleration signal from the acceleration sensor, the microcomputer normally judges whether or not a vehicle collision has occurred. When the judgment result is "YES", the microcomputer outputs trigger signals simultaneously to the first and second transistors from two output ports so that the first and second transistors are turned ON. As a result, a direct electric current is supplied to the squib from the power source to ignite the squib, thereby inflating the air bag.
In the control system thus constructed, two transistors are used, and therefore even if one of the transistors is subjected to an ON failure (i.e., one of the transistors is accidentally turned ON due to failure or malfunction thereof), the air bag can be prevented from being accidentally inflated. However, there still remains a possibility, very small though, that the air bag is accidentally inflated when both of the transistors are simultaneously turned ON by accident either due to failure of the transistors themselves or due to runaway of the microcomputer, or when the squib is short-circuited to a body of the vehicle under the condition that the transistor near the power source is accidentally turned ON due to failure of the transistor itself.
A control system disclosed in U.S. Pat. No. 5,083,276 is similar to that of the above Japanese Publication in the respect that two trigger signals are outputted simultaneously from two output ports when a microcomputer judges that a vehicle collision has occurred. The control system of this U.S. Patent further comprises two analog collision judgment circuits which output trigger signals respectively when the analog collision judgement circuits judge that a vehicle collision has occurred. The transistors are turned ON only when they receive simultaneously trigger signals from the corresponding output ports of the microcomputer and trigger signals from the corresponding analog collision judgment circuits, respectively. Owing to this arrangement, the air bag can be prevented from being accidentally inflated even at the time the microcomputer runs away. However, it is still impossible for this control system to prevent the air bag from being accidentally inflated when both of the transistors are simultaneously turned ON by accident.